The practical limits of optical lithography assume that the medium through which imaging is occurring is air. This practical limit is defined by the effective wavelength equation
            Λ      eff        =          λ              2        ·        n        ·        NA              ,where λ is the wavelength of incident light, NA is the numerical aperture of the projection optical system, and n is the index of refraction of the medium. Now, by introducing a liquid (instead of the air) between a last lens element of the projection optical system and a wafer being imaged, the refractive index changes (increases), thereby enabling enhanced resolution by lowering the effective wavelength of the light source. Lowering a light source's wavelength automatically enables finer resolution of smaller details. In this way, immersion lithography becomes attractive by, for instance, effectively lowering a 157 nm light source to a 115 nm wavelength, thereby gaining resolution while enabling the printing of critical layers with the same photolithographic tools that the industry is accustomed to using today.
Similarly, immersion lithography can push 193 nm lithography down to 145 nm. In theory, older technology such as the 193 nm tools can now still be used. Also, in theory, many difficulties of 157 nm lithography—large amounts of CaF2, hard pellicles, a nitrogen purge, etc.—can be avoided.
However, despite the promise of immersion photolithography, a number of problems remain, which have so far precluded commercialization of immersion photolithographic systems. These problems include optical distortions. For example, during immersion lithography scanning, sufficient g-loads are created that can interfere with system performance. These accelerative loads can cause a vibrational, fluidic shearing interaction with the lens resulting in optical degradation. The up and down scanning motions within the lens-fluid environment of Immersion Lithography can generate varying fluidic shear forces on the optics. This can cause lens vibrational instability, which may lead to optical “fading”. Other velocity profile non-uniformities can also cause optical distortions.